U.S. Pat. No. 4,313,035 issued to D. S. Jordan et al. on Jan. 26, 1982, describes a Method of Providing Person Locator Service in which a person desiring to receive telephone calls when he or she is away from their normal home or business location can update information stored in a centralized database, so as to link an assigned personal locator number (PLN) with a telephone number at which the person is presently located. Thereafter, if any calls are placed to the person using their assigned PLN, the calls may be forwarded to the telephone at the person's present location.
While the Jordan invention is quite useful in dealing with establishing communications in our increasingly mobile society, it suffers nevertheless from some limitations. In particular, it is necessary for the subscriber to take affirmative action in order to update the system, by placing a call to a centralized database in order to change the information stored therein that is associated with their PLN. If the person forgets or otherwise fails to make the call, the system cannot work, as the stored information will be absent or out of date. Likewise, even if the call is made, but the plans of the person traveling are changed for any number of reasons, subsequent calls to the person may be misdirected.
U.S. Pat. No. 4,901,340 issued to T. S. Parker et al. on Feb. 13, 1990, attempts to deal with a similar problem in the context of cellular mobile radiotelephone service, in which each subscriber has a designated home service area, and is considered a "roamer" when in any other areas. In order to enable completion of calls to roamers, Parker proposes a system in which the subscriber places a request at a foreign mobile telephone switching office (MTSO), indicating that the roamer desires that calls placed to his or her home service area be forwarded to a different area, normally the area in which the roamer is currently located. This request, after validation, causes a temporary directory number to be assigned to the subscriber. Subsequent calls to the home MTSO are then forwarded to the roamer at the foreign MTSO.
Here again, the system will be useful only if the subscriber takes the active step of requesting assignment of a temporary directory number in the area served by a foreign MTSO. Without this step, the system is unaware of the movement of the subscriber, and calls cannot be forwarded.